The present invention relates to a differential pressure sensor.
Transducers are widely used in measurement and control systems for converting a physical quantity, such as pressure, into a corresponding signal suitable for processing. A pressure transducer is a sensor which responds to an applied pressure and produces a signal (e.g., electrical, mechanical, or pneumatic) representative of the pressure. Typically, a pressure transducer utilizes a pressure-sensitive element that includes at least one sensor component having a position that varies with applied pressure. The position of the pressure-sensitive element is transformed into an electrical signal representative thereof.
One particular class of pressure transducers employs a peripherally supported metal diaphragm as the pressure-sensitive element and operates in response to an applied pressure to translate a physical displacement of the diaphragm into an electrical signal. A capacitive-sensing arrangement is used for the conversion from physical displacement into an electrical signal. Briefer et al., U.S. Pat. No. 5,604,315, disclose such a pressure transducer. The planar nature of the diaphragm makes it susceptible to measurement differences based on its orientation. A horizontally oriented diaphragm will experience a greater influence due to gravity than a vertically oriented diaphragm. Hence, horizontally and vertically oriented diaphragms will result in different pressure readings. Further, if the sensor is attached to a non-stationary object, such as a vibrating duct, the diaphragm will experience movement which introduces additional error into measurements made therefrom. Typically the metal diaphragm itself is supported by a metal support and the metal diaphragm tends to fatigue over time resulting in a drift of the measurements made therefrom. Accordingly, such pressure transducers need to be recalibrated, often on a monthly basis, to maintain accurate measurements.
Kavlico Corporation of Chatsworth, Calif. provides pressure transducers that include a ceramic body having on one side a circular depression of approximately three-thousand thousandths inches depth. The back inside surface of the circular depression is coated with a conductive metal surface. A first wire is connected to the conductive metal surface. Covering the circular depression is a ceramic diaphragm with a conductive metal coating that is electrically isolated from the conductive metal surface. The exterior side of the diaphragm is coated with a thin layer of ceramic. This configuration forms an enclosed cavity between the ceramic layer and the metal surfaces. A second wire is connected to the metal on the ceramic diaphragm. A housing encloses the pressure transducer and isolates both sides of the pressure transducer from one another. Together, a first exterior fitting of the housing and a hole in the ceramic body allows air flow to and from the enclosed cavity, and hence may exert pressure on one side of the ceramic diaphragm. A second exterior fitting of the housing allows air flow to and from the other side of the ceramic diaphragm, and hence may exert pressure thereon. The combination of the first and second fittings permit a differential pressure to be applied to the ceramic diaphragm which moves the diaphragm in accordance with the pressure differential. The first and second wires are used to measure the change in capacitance between the conductive metal surface and the metal on the ceramic diaphragm that occurs in accordance with any change in pressure differential. Similar to the class of pressure transducers discussed above, the Kavlico sensor is susceptible to gravity induced measurement differences based on orientation, vibrational effects when attached to non-stationary objects, and the ceramic diaphragm tends to fatigue over time resulting in drift of the measurements made therefrom. Accordingly, such pressure transducers need to be recalibrated, often on a monthly basis, to maintain accurate measurements. Such calibration is time consuming and expensive. Further, the dynamic range of such a pressure transducer, which is the range of measurements about a nominal value, is typically about 10 percent change in capacitance.
What is desired, therefore, is a pressure transducer that is substantially insensitive to gravity, vibration, and fatigue. Further, the pressure transducer should have a significant dynamic range, such as 100 percent change in capacitance.